Segmentation Facts

Segmentation Facts

LAN segmentation is the process of dividing the network to overcome problems such as excessive collisions, broadcast traffic, or heavy network traffic. By segmenting a LAN, you can increase network performance, maximize bandwidth, and reduce congestion.

As you segment the network, you will need to consider the collision and broadcast domains on the network.

• A collision domain is any network or subnetwork where devices share the same transmission medium and where packets can collide. Collisions naturally increase as the number of devices in a collision domain increase.
• A broadcast domain is any network or subnetwork where computers can receive frame-level broadcasts from their neighbors. As you add devices to a network segment, the amount of broadcast traffic on a segment also increases. Note: A special condition called a broadcast storm happens when broadcast traffic is sent, regenerated, and responded to. In this condition, the amount of broadcast traffic consumes network bandwidth and prevents normal communications. Faulty devices or improper configuration conditions can lead to a broadcast storm.

Segmentation may increase the number of both the collision and broadcast domains. Membership within collision or broadcast domains differs depending on the connection device used.

Device	Collision Domain	Broadcast Domain
Hub	All devices connected to the hub are in the same collision domain.	All devices are in the same broadcast domain.
Bridge or Switch	All devices connected to a single port are in the same collision domain (each port is its own collision domain).	All devices connected to the bridge or the switch are in the same broadcast domain.
Router	All devices connected to a single interface are in the same collision domain.	All devices accessible through an interface (network) are in the same broadcast domain. Each interface represents its own broadcast domain if the router is configured to not forward broadcast packets.

In considering a network expansion solution, it is important to identify the connectivity problems you need to resolve, and then identify the device that is best suited for that situation. The main differences between routers, switches, and bridges are the range of services each performs and the OSI layer at which they operate.

Device	Characteristics
Router	Routers perform the following functions that are not performed by bridges or switches. Route packets between separate networks Modify packet size through fragmentation and combination Route packets based on service address Choose a router if you need to: Connect your network to a WAN, such as the Internet Filter broadcast traffic to prevent broadcast storms Connect two separate networks that use the same protocol Improve performance in the event of a topology change (routers recover faster than bridges or switches) Reduce the number of devices within a domain (effectively increasing the number of broadcast domains) Enforce network security Dynamically select the best route through an internetwork Connect two networks of different architectures, for example Ethernet to Token Ring
Switch	Choose a switch if you need to: Provide guaranteed bandwidth between devices Reduce collisions by decreasing the number of devices in a collision domain (effectively creating multiple collision domains) Implement full-duplex communication Connect two network segments or devices using the same protocol Provide improved performance over a current bridged network Switch traffic without the cost or administration involved with routers
Bridge	Choose a bridge if you need to: Isolate data traffic to one network segment Route traffic from one segment to another (with the same network ID) Link unlike physical media (e.g. twisted pair and coaxial Ethernet) of the same architecture type Link segments that use the same protocol Create segments without the expense and administration of routers Note: In most cases where you might use a bridge, choose a switch instead.

In general, follow these guidelines to make decisions about the appropriate connectivity device.

• Use a bridge to segment the network (divide network traffic) and to provide fault tolerance.
• Use a switch to reduce collisions and offer guaranteed bandwidth between devices.
• Use a router to filter broadcast messages, implement security, or connect different networks.

LAN segmentation and design may be affected by the types of applications and protocols running over the network. For instance, Voice over Internet Protocol (VoIP) requires a well-engineered, end-to-end network that provides little latency for data stream transmission. Fine-tuning the network to adequately support VoIP involves overcoming the following challenges:

• VoIP requires a very low delay as data is transferred between the sending and receiving phones, e.g. less than 200 milliseconds (.2 seconds).
• During transfer, the jitter (variations in delay) must be low as well, e.g. less than 30 milliseconds (.03 seconds).
• When packets do not arrive at the destination it is known as packet loss. If a VoIP packet was lost in transit, there is no need to recover the packet. This is because by the time the packet is recovered, it would sound like a break in the sound of the VoIP call.
• Echo is hearing your own voice in the telephone receiver while you are talking. When timed properly, echo is reassuring to the speaker; if the echo exceeds approximately 25 milliseconds, it can be distracting and cause breaks in the conversation. VoIP implementations use echo cancellers to regulate the echo.
• To secure VoIP data, the network should have a VoIP Virtual Private Network (VPN) solution. A VPN is a network that uses encryption to allow IP traffic to travel securely over the TCP/IP network. Without a VoIP VPN solution, it is relatively easy to eavesdrop on VoIP calls and even change their content.
• In some cases, IP telephones require Power over Ethernet (PoE). PoE is useful for powering IP telephones and other appliances where it would be inconvenient, expensive, or infeasible to supply power separately.